What are common causes for Blend files growing in size?
In each instance the cause of growing file size may be different.

Common causes include:


Groups of objectsCause: Objects in a group might not exist in any scene.Solution: Check groups in the outliner, if you want you can add an empty scene and RMB in the outliner to instantiate all group objects in the scene to check all the objects you have in groups.
Physics cacheCause: Objects with internally stored physics cache may accumulate cache in the blend file.Solution: Clear the cache for your physics objects, you may want to use external cache files too.
Auto-packed texturesCause: Texture auto-pack is a feature, stored per-blend file, that will pack all your textures when saving.Solution: Check that you have auto-pack disabled in the menu: "File -> External Data -> Automatically pack into .blend".




feel free to suggest extras in comments - or add own answer :)